Hemodynamics of cerebrovascular spasm.

An understanding of the hemodynamics of cerebrovascular spasm following subarachnoid hemorrhage is important for the diagnosis and treatment of this potentially dangerous condition. An overview model is presented which includes the main elements determining the overall effect of vasospasm. The model included realistic pressure-flow-velocity-diameter relationship encountered in a geometry resembling that of vasospasm of the middle cerebral artery. Viscosity was adjusted to that expected of human blood. Furthermore, a realistic model the cerebral autoregulation was included. The effects of induced hypertension as well as hypotension were studied. It was found that the friction pressure loss in the spastic segment was 3.5 times as high as that predicted by using the Hagen-Poiseuille formula. The reason for this discrepancy was probably the 'inlet length effect' considerably increasing the friction. Furthermore, including the Bernoulli kinetic pressure energy, a formula was proposed that accurately described the experimental data. From this hemodynamic perspective, strong support was found for the present trend to use aggressive hypertensive therapy in patients with vasospasm. The results also confirmed that TCD velocity measurements in the spastic segment when taken alone may not be a good index of the degree and effect of the spasm. These measurements must be combined with other techniques such as extracranial Doppler or CBF to assess the degree of spasm.